Substrate issues

1. the 2-layer PCB i am trying to model has no ground plane. hence i simulate the model with 2 conducting layers but no GND layer. will this affect the simulation accuracy?2. is it possible to define 2 different substrates in the same layer? say for example, a small 1 mm x 1mm square is made of FR4 but it is enclosed by a bigger 4mm x 4mm alumina3. it seems that the dielectric layers go off to infinity. is it possible to limit their area and let everything else be freespace? say just a 4mm x 4mm area of alumina and freespace everywhere elese? how can this be done?